The present invention relates to a pattern forming material and, more particularly, to a silicone based negative resist, and to a method for forming a submicron pattern having a high aspect ratio using such a resist.
In the manufacture of ICs and LSIs, organic compositions containing a polymeric compound and called a resist are frequently used. More specifically, a substrate to be processed is covered with a selected organic composition as a resist film. The resist film is irradiated with a high-energy beam in a predetermined pattern to form an electrostatic latent image in the resist film. Development is then performed to form a patterned resist film. Thereafter, the substrate is immersed in an etching solution to chemically etch the exposed portion of the substrate which is uncovered by the resist pattern, or to dope an impurity therein.
However, with a recent tendency toward higher integration of IC circuits, further minimization is desired. In wet etching methods using an etching solution, the problem of side etching is inevitable. In view of this problem, dry etching such as reactive ion etching using a gas plasma is becoming popular. However, conventional resist materials are themselves etched during dry etching of the substrate; thus the resist film must be thick to allow selective etching of the substrate. For this reason, a resist material with a high dry etching resistance has been desired. However, a resist material which is satisfactorily resistant to dry etching has not yet been proposed.
Meanwhile, in order to provide a multilayered wiring layer or a semiconductor element of a three-dimensional array structure, it is desired to form a resist pattern on a nonplanar substrate. The resist film must be made thick to level a step.
Furthermore, in order to trap highly accelerated ions before they reach the substrate, the resist film must again be thick. However, with a conventional resist material, resolution is lowered as the film thickness increases, preventing formation of a fine pattern.
In order to solve this problem, a method has been proposed wherein a resist is applied in a multilayered structure to form a resist pattern having a high aspect ratio. According to such a method, a thick film of an organic polymeric material is formed as a first layer. A thin resist material film is formed thereover as a second layer. The second, resist layer is selectively irradiated with a high-energy beam and is developed. The resultant pattern is used as a mask to dry etch the organic polymeric material of the first layer and to form a pattern of a high aspect ratio. With this method, when a conventional resist material is used for the second layer, the ratio of dry etch rates of the materials of the first and second layers, that is, the selectivity, cannot be set to be high. Akiya et al (article to be published in 43rd Proceedings of the Japan Association of Applied Physics, p. 213) reported that when a multilayer pattern consisting of a first layer of polymethyl methacrylate (to be referred to as PMMA hereinafter) and a second layer of chloromethylated polystyrene is dry etched using carbon tetrachloride as an etching gas, the selectivity can be selected to be very high and a resist pattern having a high aspect ratio can be formed. However, in this case, the etch rate of the PMMA is also low; it takes a long time to etch a thick PMMA film. Carbon tetrachloride as an etchant also etches the underlying substrate.
A three-layered structure has also been proposed as a multilayered resist pattern obtained using an oxygen plasma, and consists of a first, thick layer of an organic polymeric material, a second layer of a resist, and a third layer which consists of an inorganic material having a high resistance to the oxygen plasma and formed between the first and second layers. In this case, the pattern of the resist material is used as a mask to dry etch the inorganic material using a gas such as carbon tetrachloride, carbon tetrafluoride or argon. Subsequently, using the resultant inorganic material pattern as a mask, the organic polymeric material layer is dry etched using oxygen. With this method, the oxygen plasma can quickly etch the first, thick organic polymeric material layer and the substrate is not etched at all. Accordingly, a resist pattern of a desired profile may be formed without requiring monitoring of the end timing of etching. However, this method requires a large number of steps.
When a silicone based resist having a high resistance to dry etching using an oxygen plasma is used for the second layer, an oxygen plasma can be used to dry etch the organic polymeric material of the first layer, using the resist pattern of the second layer as a mask. For this reason, a resist pattern of a high aspect ratio can be formed within a short period of time and with a small number of steps. However, a currently available silicone based resist has a glass transition temperature considerably lower than room temperature. A conventional silicone based resist having a low molecular weight is in liquid or milky form, is difficult to handle, and has a low sensitivity to high-energy radiation.
On the other hand, a conventional silicone based resist having a high molecular weight is in a rubbery form, is easier to handle, and has a higher sensitivity to high-energy radiation. However, the resultant pattern frequently has an undulation due to swelling in a developing solvent, resulting in a low resolution. Furthermore, a functional group having a high chain reaction property such as a vinyl group is introduced to improve sensitivity of the cross-linking reaction. This also results in a low resolution.